Keeping an urban planter box stable on a windy rooftop is mostly about weight, wind exposure, and secure anchoring. Start with a wide, heavy planter rather than a tall, narrow one; a low center of gravity makes tipping much harder. Fill it with compacted soil, and add a layer of gravel or a few concrete pavers at the bottom if the rooftop can safely take the load. Place planters against a solid wall or parapet where wind is lower, but avoid blocking drainage, vents, or emergency access.
Use ballast such as sandbags, paving slabs, or water-weighted bases to hold the box down, provided you check the rooftop's load limit. Fix the planter to a sturdy, purpose-built frame or anchor bracket attached to the building structure, not just the waterproof membrane. Never drill through the roof yourself; ask a professional to install and seal any anchors. For renters, a non-penetrating ballasted system is usually safer. Add a windbreak such as a perforated screen or a hardy hedge, and choose low-growing, wind-tolerant plants. Finally, inspect straps, ties, and drainage after storms, and move lightweight pots indoors during severe wind alerts.